1. Field of the Invention
The present invention relates generally to a hydrostatic anti-vibration system for suppressing vibration of a construction utilizing motion of a hydraulic fluid filled in a tank. The invention also relates to a method for adjusting an active vibration frequency range to a natural period of the actual construction.
2. Description of the Related Art
Such type of hydrostatic anti-vibration system has been proposed in commonly owned Japanese Unexamined Patent Publication No. 5-60173. A brief discussion will be given of the prior proposed hydrostatic anti-vibration system with reference to FIG. 8. As shown in FIG. 8, the hydrostatic anti-vibration system generally comprises a tank 103 having a lower tank portion 101 of essentially rectangular configuration and hollow vertical extensions 102 extending upwardly from respective of the four corners of the lower tank portions 101. Water as an anti-vibration medium fills up the entire volume of the lower tank portion 101 and further fills the hollow vertical extensions 102 for approximately half of their heights with upper air containing spaces maintained thereabove. The air containing spaces of respectively adjacent hollow vertical extensions 102 are communicated through four ducts 105. At intermediate portions of the four ducts, intermediate tank portions 106 are defined. The intermediate tank portions 106 are filled with water to a level substantially equal to the level of water in the hollow vertical extensions 102.
Partitioning plate 107 depending from the ceilings of the intermediate tank portions 106 extend into the interior space of each intermediate tank for dividing the upper portion of the interior space of the intermediate tank, thereby defining substantially U-shaped water communication paths. Rotary shafts 108 are rotatably mounted on the lower ends of respective partitioning plate 107 in horizontal orientation along the lower edges of the partitioning plates. Pivotal plates 109 are secured on the rotary shafts 108 for pivotal movement with the rotary shafts in response to flow of the water contained in the intermediate tank portions 106. Coil springs 110 are provided between the partitioning plates 107 and the rotary shafts 108. The coil springs 110 exert a biasing spring force to rotary shafts 108 for restricting rotary motion thereof and thereby restricting pivotal motion of the pivotal plates 109. By appropriately adjusting the spring coefficients of the coil springs 110, resistance against pivotal motion of the pivotal plates 109 can be adjusted depending upon the natural period (natural frequency) of the construction of which vibration is to be suppressed.
The hydrostatic anti-vibration system is installed on the roof or top of the construction. According to vibration induced on the construction, the water in the tank 103 causes rocking motion. In this rocking motion, due to inertia moments of the water, water collides on the interior walls of the tank 103 with a delay to the natural period or vibratory phase of the vibration of the construction. According to the rocking motion of water in the tank 103, the water level in each hollow vertical extension 102 is varied, thus increasing or decreasing the air pressure therein. Variation of the air pressure in the air containing spaces of two adjacent of the hollow vertical extensions 102 is introduced to opposite sides of the substantially U-shaped water flow path in the respective intermediate tank portion 106, thus causing rocking water flow in such intermediate tank portion 106 along such U-shaped flow path. Against the water flow thus induced, the spring biased pivotal plate 109 serves to provide resistance. This resistance cancels vibratory energy of the construction. With the construction set forth above, high anti-vibration effect can be attained for horizontal bi-directional vibration with a relatively small size of the tank. Furthermore, such hydrostatic anti-vibration system is advantageous in comparison with a pendulum type anti-vibration system by requiring no externally exposed mechanically movable component. Furthermore, the tank may serve as a reservoir for drinking water in the case of emergency.
In the case where such anti-vibration system is desired to operate as a hybrid type anti-vibration system by adding an active driving device for a passive type operation system for suppressing vibration of the construction more effectively, it becomes necessary to make the natural frequency of the anti-vibration system consistent with the natural period of the construction. In practical implementation, the natural frequency of the hydrostatic anti-vibration may be determined on the basis of an approximated value of the natural period of the construction derived from calculation of construction of the primary body thereof. Then, on the basis of such natural frequency, the dimensions and volume of the tank, the amount of water to be filled and so forth are designed for the anti-vibration system. The anti-vibration system is thus constructed on the roof or in the vicinity of the top of the construction, according to the design thereof.
However, certain features of such prior proposed hydrostatic anti-vibration system need to be improved. Namely, in the construction set forth above, since the hollow vertical extensions 102 are provided at the four corners of the lower tank portion 101 and four intermediate tanks 106 are provided for communicating between the four hollow vertical extensions, the construction is complicated. Furthermore, it is possible to cause disturbance of the flow of the water by horizontal bi-directional composite vibrations.
Also, since in such prior proposed construction the respective upper spaces of the intermediate tank portions 106 separated by the partitioning plates 107 are communicated with respective corresponding adjacent air containing spaces of the hollow vertical extensions 102, when a fluid force is generated in the direction indicated by the arrow in FIG. 8, for example, compression of the air is caused at the air containing space of one of the hollow vertical extensions 102 (left side in the illustrated case). Compressed air is then introduced into the communicated side of the upper space of the intermediate tank portion 106. The air pressure thus introduced can serve to suppress water flow in the intermediate tank portion 106 induced by the vibration of the construction. In such case, both fluid forces serve to cancel each other, thus causing the anti-vibration system to be not effective in suppressing vibration of the construction.
In addition, since water in the tank 103 and water in the intermediate tank portions 106 are separated completely, each such respective water portion has to be managed independently of the others, thus causing management of the water levels to be cumbersome. Furthermore, water in the hollow vertical portions 102 of the tank 103 and water in the intermediate tank portions 106 may flow from one to the other when large amplitude vibration occurs. In such case, the water levels in the hollow vertical extensions and in the intermediate tank portions can be varied to cause variation of the anti-vibration characteristics. Therefore, at every occasion of variation of the water levels in the hollow vertical extensions and in the intermediate tank portions, adjustment of the water levels becomes necessary to maintain the desired anti-vibration characteristics. Furthermore, the natural period of the construction varies delicately depending upon variation of the weight of the construction due to interior construction, modification of the construction design, changing of layout and so forth, in practice. Therefore, in the sense of high precision, the natural period of the construction cannot be determined until completion of construction. Furthermore, even after completion of construction, the weight of the construction and weight distribution therein are variable depending upon conditions of use of the construction. Such variation of the weight or weight distribution of the construction may cause variation of the natural period. Thereby, the natural frequency of the anti-vibration system, that is designed based on that of the construction, may be out of the effective range in terms of anti-vibration effect. As a solution for this, it may be possible to design the anti-vibration system for accommodating such future possible variation of the weight and/or weight distribution of the construction by adjustment of the water amount and the internal pressure in the tank. However, even with this measure, adjustment of the water amount and internal pressure is very troublesome in practice.